High purity polycrystalline silicon is used in the shape of wafers for semiconductor devices and solar cells. Polycrystalline silicon of a lower purity than semiconductor grade silicon that is described in the art as upgraded metallurgical grade (UMG) silicon can be used to produce solar cells. The tradeoff for lower purity is less efficient solar cells at a lower initial cost since silicon does not have to go through as many refining stages from its naturally found metallic or silicon oxide state.
One method of purifying silicon is disclosed in U.S. Pat. No. 7,197,061. In that patent a material, such as silicon, can be melted and directionally solidified in a vessel by utilization of electric induction power. Separate induction coils are provided along the exterior height of the vessel so that electric power can be sequentially removed (starting from the bottom of the vessel) once all material in the vessel has been melted. Initially all induction coils are powered from an alternating current (ac) source to melt all silicon in the vessel; silicon has a nominal melting point of 1,410° C. In this process, known as directional solidification or heat exchanger process, impurities in the silicon melt move to the top of the silicon melt/solid interface boundary as the silicon melt directionally solidifies from the bottom of the vessel. At completion of total directional solidification of the melt, a solid section of highly impure silicon is at the top of a solid section of highly purified silicon. The patent also discloses circulating a cooling medium in each induction coil after electric power is removed to hasten solidification of the silicon melt.
Other techniques involve utilizing a heat removal source, such as a chill plate, below the bottom of the vessel after all the silicon in the vessel has been melted and the heating source has been removed. A disadvantage of this method is that the rate of vertical movement of the silicon melt/solid interface during the directional solidification process is slow because of the application of the heat removal source from the bottom of the vessel.
It is one object of the present invention to provide apparatus for, and method of, directionally solidifying a material in a vessel with an improved solidification rate over that achievable with a bottom heat removal source.
It is another object of the present invention to provide apparatus for, and method of, producing a purified material in a vessel by directional partial solidification (cooling) of the material to the nominal amorphous-to-crystalline transition (solidification) temperature of the material, and then re-melting the purified material for either further purification by directional solidification or withdrawal from the vessel as a molten material.